This invention relates to an arrangement for cooling supply air in an air-conditioning installation, comprising
a heat transfer device for transferring heat between supply air and exhaust air by means of heat transfer surfaces; and PA1 a humidifying device for humidifying the heat transfer surfaces on the exhaust air side. PA1 t.sub.A =entry temperature of supply air=27.degree. C. PA1 t.sub.B =surface temperature on exhaust side=17.degree. C. PA1 (t.sub.A and t.sub.B =temperatures at points A and B) PA1 the heat transfer surfaces on the side of supply air and exhaust air, respectively, are separated into separate heat exchangers; and PA1 that the heat transfer surfaces are selected in such a way that the surface temperature of the heat transfer surfaces on the exhaust air side is close to the dew-point temperature of exhaust air.
Efforts made to reduce the use of freons have compelled the air-conditioning industry to look for alternative cooling systems in place of compressor cooling utilizing freons. The best-known of such systems is the so-called indirect evaporative cooling described e.g. in Finnish Patent Specification 67 259. In this method, exhaust air is humidified by an evaporative humidifier positioned in an exhaust duct, whereby water binds heat as it evaporates so that the temperature of exhaust air drops close to the saturation point. "Coldness" in the cooled exhaust air is recovered into supply air by a heat exchanger presently used widely in the recovery of heat in winter; in other words, supply air is cooled.
A drawback of the system is its limited cooling power. Especially when exhaust air and/or outdoor air is warm and moist, the cooling power is not sufficient. This is due to the fact that moist air is not able to receive any greater amounts of water steam, and so its saturation temperature is high. In addition, evaporative humidifiers are not usually able to humidify air up to the dew point. The humidification ratio has been defined as the mass ratio between the amount of water that in theory can be evaporated into air and the amount of water that actually is evaporated. The humidification ratio of the best evaporative humidifiers ranges between 80 and 90%. In addition to this, the cooling power is reduced by the efficiency of the heat exchanger, which is usually defined as the ratio of the temperature drop of supply air to the difference between the initial temperatures of supply air and exhaust air. This ratio, called temperature efficiency, is between 70 and 80% for the best air/air heat exchangers. As a whole, the ratio between the actually achievable cooling power and the theoretical cooling power usually remains below 70%. Not even the theoretical power would be sufficient in all cases.
Various attempts have been made to compensate for the limited cooling power. The simplest way is to provide the missing power by the use of compressor cooling, which, however, involves considerable investment and operating costs. Even though the use of freons is reduced, it cannot be totally avoided.
Another approach is described in Finnish Patent Specification 88431. Additional cooling is accomplished by the use of cold tap water before the water is passed into the network of water pipes in the building. A drawback of this method is that in many cases the tap water consumption of buildings is so low and varies to such an extent that it is able to meet the need of additional cooling only in a limited number of buildings. Overflowing cooling water into the drain easily raises the operating costs unreasonably. Moreover, an additional piping has to be provided in the building for cold drinking water, which increases the investment costs. For this reason, tap water can be used to cut down load peaks only in some buildings.
Still another approach is described in Finnish Patent Specification 57 478. Instead of using a separate humidifier, air is humidified by allowing humidifying water to run onto the heat transfer surfaces on the delivery side of a plate heat exchanger. In this patent specification, in an attempt to increase power, 2-step cooling is used, where so-called auxiliary air, which may consist of exhaust air or supply air, is first cooled in an auxiliary heat exchanger and then humidified and used for cooling supply air.
Due to its high investment and operating costs, the 2-step cooling has not been used widely. In order that real benefit could be derived, an extra heat exchanger is needed as well as a blower or the like for drawing the auxiliary air flow through the heat exchanger. Air cooling as such will not give the desired result, as the mass ratios vary. It may be used successfully when the amount of exhaust air is about 2 times greater than the amount of supply air for some special reason. In addition, the cooling power of the second step is substantially lower than that of the first step.
Instead, humidified heat exchangers have been built to some extent. In theory, they should operate in such a way that the temperature of a moisture film on the outer surface of the heat exchanger will coincide with the saturation temperature of air. The air treatment process should, in theory, take place as illustrated by an exemplifying curve in the h-x diagram of FIG. 1, i.e. in the following way:
The temperature of supply air=outdoor air is assumed to be 27.degree. C., and the relative humidity 40%, point A in FIG. 1. The temperature of exhaust air=room air is assumed to be 24.degree. C., and the relative humidity 50%, point B in FIG. 1. In theory, the surface of the heat exchanger on the exhaust air side should be at its dew point, that is, at a temperature corresponding to a relative humidity of 100%, i.e. 17.degree. C., point C in FIG. 1. After a separate evaporative humidifier, the temperature of exhaust air would be slightly above the humidification ratio, with a humidification ratio of 0.88, for instance, about 18.degree. C., point C' in FIG. 1. The difference is thus 1.degree. C.
As one surface of the heat exchanger is, in theory, at the dew-point temperature, there are no surface resistances in the heat transfer on this side. In other words, there should occur a decisive improvement in the heat transfer coefficient and thus in the temperature efficiency. The relatively complicated theory of heat transfer will not be explained herein, but it may suffice to mention that an efficiency of 0.7 corresponding e.g. to dry heat transfer should be improved to about 0.84. The corresponding temperature drop of supply air is EQU .DELTA.t.sub.s =.eta..sub.A (t.sub.A -t.sub.B),
where .eta.=temperature efficiency 0.84
that is EQU .DELTA.t.sub.s =0.84(27-17)=8.4.degree. C.
The final temperature of supply air would thus be t.sub.D =27-8.4=18.6.degree. C. (point D, FIG. 1). The power available for cooling is represented by the difference between the entry temperature of exhaust air and the final temperature of supply air, i.e. EQU .DELTA.t.sub.j =24-18.6=5.4.degree. C.
In a system employing a separate humidifier the following applies: EQU .DELTA.t'.sub.s =0.7(27-18)=6.3.degree. C.
and the final temperature of supply air t.sub.D =27-6.3 =20.7.degree. C. (point D' in FIG. 1).
The power available for cooling: EQU .DELTA.t'.sub.j =24-20.7=3.3.degree.C.
The power available for cooling (for removing thermal loads) would thus increase really significantly, that is, in the ratio EQU .DELTA.t.sub.j /.DELTA.t'.sub.j =5.4/3.3=1.63.
The power increase thus seems to be considerable.
In practice, however, this does not hold true. It has been found that the final temperature of supply air remains clearly above the value to which it should drop according to the above theory. In practice, the power increase is only 10 to 25% as compared with a separate humidifier. In fact, the change of state of exhaust air takes place in the direction of a curve drawn by broken line in FIG. 1, towards a temperature above the dew point. This is due to the fact that heat transfer and evaporation are dynamic processes, which typically take place at a specific speed.
On examining the moisture film on the surface of the heat exchanger, it can be seen that heat is introduced into the film from supply air and removed from it into exhaust air partly through conduction, partly bound in the evaporating water steam. In order that the temperature of the moisture film would coincide with the dew point, heat should be bound merely in the evaporation of water steam. In practice, this does not happen as the surface that evaporates water is too small. As compared with e.g. the humidification cells of the above-mentioned evaporative humidifiers, the area of the moisture film formed on the plate heat exchanger is only a fraction. The evaporation rate from the too small liquid surface is simply too low to maintain the moisture film at a temperature corresponding to the dew point. The heat transfer takes place partly through conduction, and therefore the change of state takes place along a curve drawn by broken line in FIG. 1.
The plate heat exchanger disclosed in Finnish Patent Specification 57 478 also has another drawback. Bacterial contamination easily occurs on moist surfaces within the above-mentioned temperature range. As plate heat exchangers are large in size, it is difficult to make them water-proof; in practice, minor leakages occur as a result of corrosion, thermal expansion, vibration, pressure variation, etc. Water seeping to the supply air side evaporates, whereby possible bacteria become encapsulated and are entrained in the supply air. The resulting health hazard is so severe that humidified plate heat exchangers have been totally abandoned.